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An astronomer says experts need to expand their definition of Earth-like exoplanets

A leading scientist in the field of exoplanet atmospheres authored a study in the recent issue of Science that urges researchers to broaden popular conceptions of what constitutes habitable, Earth-like planets.

Sara Seager, a theoretical physicist with the Massachusetts Institute of Technology (MIT), believes that research is focusing too heavily on finding habitable planets that follow Earth-like patterns, including proximity to the sun and abundance of rocky surface area. Instead, Seager posits a focus on the makeup of alien atmospheres?specifically their greenhouse gasses.

?[W]hat primarily controls surface temperature is the greenhouse effect, what types of gases are in the atmosphere, and how massive a planet?s atmosphere is,? commented Seager. ?That?s what we really have to understand.?

One example of a potentially habitable exoplanet, despite being much further away from its parent star than Earth, might have enough hydrogen in its atmospheres to sustain both liquid water and life forms. The distance from the star would not be an issue since the greenhouse power of hydrogen would be strong enough to theoretically warm the planet?s surface.

When encountering particularly dry planets that are closer to their suns, Seager stated that less water would be beneficial as moisture levels are an extremely effective greenhouse gas. She used Venus as an example; once heavy in water, the moisture level accelerated greenhouse gas effects to the point of making the planet uninhabitable.

Even ?rogue planets,? those exoplanets that travel the stars without a sun of their own, could be viable settings for life if they exhibit certain traits, including heat from radioactive cores. ?I hope it gets people to realize that so many types of worlds could be habitable, and that our chance of finding one is higher when we accept that,? said Seager in an interview with SPACE.com.

James Kasting of Pennsylvania State University agrees that Seager?s theories are commonly held among many in the exoplanet community. Studying the necessary aspects of exoplanetary atmospheres, however, is a tough issue to address.